The world of the very small is about
to receive a very powerful engine. Berkeley Lab scientists have created
the world's smallest electric motor that may someday power nanoscale devices
that walk, crawl, swim, and fly.

Attach nanosized legs or wings
to this engine, and tomorrow's nanobots could be as fast and nimble
as a housefly. (Courtesy Zettl Research Group)

Although it is too early too determine what the motor will propel 
perhaps probes that deliver disease-fighting drugs inside the body or
winging nanobots that sniff out explosives  it packs a big kick in its
tiny frame. The motor measures only 200 nanometers long (a nanometer is
one-billionth of a meter), but its power density is 100 million times
greater than that of a 225-horsepower V6 engine. It draws its enormous
power from surface tension, the same cohesive force between liquid molecules
that allows bubbles to form and insects to walk on water.

"Surface tension becomes more important as objects become smaller,
and at the nanoscale, it dominates," says Chris Regan of Berkeley
Lab's Materials Sciences Division, who developed the motor with fellow
Materials Sciences researchers Shaul Aloni, Kenneth Jensen, and team leader
Alex Zettl. Aloni, Regan, and Zettl are also scientists in the University
of California at Berkeley's Department of Physics, where much of the work
was conducted.

Their device is based on carbon nanotubes, which are hollow cylinders
of pure carbon about ten thousand times smaller than the diameter of a
human hair. Last year the Berkeley Lab scientists developed a way to move
indium particles along the tube, like auto parts on an assembly line.
Their nanoscale conveyor belt can be aimed anywhere scientists want to
deliver mass atom-by-atom, the makings of a formidable nanoassembly tool.

Now the same team has converted the conveyor belt into a tiny motor with
Herculean strength. To build the engine, two molten indium droplets, one
big and one small, are positioned side by side on a carbon nanotube. Next,
an electric current is sent through the nanotube, which causes individual
indium atoms to shuttle from the large droplet to the small droplet. The
small droplet grows until it touches the large droplet  and then surface
tension takes over. In less than a nanosecond, all of the small droplet's
atoms are transferred to the large droplet, and the small droplet collapses
to nothing.

"It is like an energetic catastrophe when the two beads meet,"
says Reagan. "We pump a lot of energy into the system, and then it
is released very quickly."

These computer-generated stills
illustrate the world's smallest motor in action. The large droplet
donates atoms to the smaller droplet, which then grows, touches the
larger one, and implodes. (Courtesy Zettl Research Group)

After the collapse, the cycle begins anew. The large droplet once again
donates atoms to its smaller neighbor. And once again, as soon as the
two droplets make contact, the small droplet relinquishes its atoms and
implodes.

This repetitive build-up followed by sudden collapse mimics the same
two-speed, back-and-forth motion that many animals use to run, swim, and
fly. For example, a fly doesn't fly like an airplane. It flies because
its wings move faster in one direction than the other direction, which
creates a vortex that produces lift. And a swimmer moves through the water
because the stroke that pushes her forward is more powerful than her recovery
stroke. Likewise, if the motor's slow-fast motion can be transferred to
nanosized legs, flippers, and wings, then nanoscale devices could be as
quick and nimble as a housefly or brook trout.

A pint-sized package of power

"Our motor is also ideal for locomotive applications because it is
very strong for its size," says Regan.

Its strength comes from the fact that the motor's power stroke, which
occurs when the small droplet collapses, is driven by surface tension.
This force becomes more important as objects become smaller, which is
why insects can manipulate surface tension as easily as a person dribbling
a basketball. Some insects can drag air bubbles underwater to help them
breathe, which is like a person holding ten gallons of water in his bare
hands.

In other words, surface tension is one million times more important for
millimeter-scale insects than it is for meter-scale humans. It is even
more important at the micron scale, which is why scientists harness it
to assemble microelectromechanical devices that measure one-millionth
of a meter. And at the nanoscale, surface tension reigns supreme. The
Berkeley Lab team's motor dissipates 20 microwatts of power when surface
tension forces the smaller droplet to collapse. This represents a power
density that dwarfs that of human-scale engines.

In addition, the motor's speed can be manipulated by changing the voltage
of the DC current that sends indium atoms scurrying from one droplet to
the other. The lower the voltage, the slower this transfer occurs and
the slower the droplets' oscillations. Higher voltages could increase
the motor's frequency into the megahertz range, meaning the droplets oscillate
more than one million times per second. Generating such frequencies with
DC electrical power means the motor can be powered by a battery or solar
cell without additional high-frequency circuitry, making it ideal for
mobile applications.

The motor is also the first oscillator with a relaxation phase driven
by surface tension. Relaxation oscillators are systems characterized by
two time scales: a slow build-up of tension followed by a rapid release.
A dripping faucet is one such relaxation oscillator. In its case, surface
tension is important during the build-up phase of the water droplet. It
restrains the slowly growing droplet, until the tension suddenly gives
way to gravity. In the nanoscale motor, however, electricity drives the
build-up of energy, and surface tension drives its rapid release.

"It's fast, simple, and very powerful," says Regan. "It
also represents a world record in terms of synthetic mechanical intricacy.
We have two moving parts within a box that is 200 nanometers on its side.
No manmade machine is this small."

This project was funded by the National Science Foundation and the Department
of Energy.